1. Field of the Invention
This invention relates to processes for depositing compounds by means of chemical vapor deposition and, more particularly, to processes for depositing silicon dioxide layers using ozone and tetraethylorthosilane as precursor compounds.
2. State of the Art
Doped and undoped silicon dioxides, which are commonly referred to as silicate glasses, are widely used as dielectrics in integrated circuits. Although silicon dioxide possesses a tetrahedral matrix which will impart a crystalline structure to the material under proper heating and cooling conditions, the silicon dioxides used as dielectrics in integrated circuits are typically amorphous materials. This application uses the term xe2x80x9csilicate glassxe2x80x9d to refer to silicon dioxides deposited via chemical vapor deposition (CVD), as the term encompasses materials containing not just silicon dioxide, but dopants and other impurities as well.
Chemical vapor deposition of silicate glasses has become of paramount importance in the manufacture of contemporary integrated circuits. For example, silicate glass doped with both boron and phosphorous is widely used as an interlevel dielectric and as a getter material for mobile sodium ions.
Chemical vapor deposition (CVD) of silicate glasses by the semiconductor industry is most commonly accomplished by reacting tetraethylorthosilane (TEOS), silane or disilane with an oxidizer. Silane is typically reacted with diatomic oxygen (O2) or nitrous oxide (N2O) at a temperature of about 400xc2x0 C. TEOS, on the other hand, is generally reacted with either O2 or ozone (O3). If a low reaction temperature is desirable, the use of ozone permits a reduction in the reaction temperature to about half that required for O2. For the sake of brevity, glass layers deposited from the reaction of O3 and TEOS shall be termed xe2x80x9cozone TEOS glassesxe2x80x9d. The reaction temperature may also be reduced for the TEOS-O2 reaction by striking a plasma in the deposition chamber. Glasses deposited via this plasma-enhanced chemical vapor deposition (PECVD) method shall be referred to hereinafter as PECVD-TEOS silicate glasses. The plasma generates highly reactive oxygen radicals which can react with the TEOS molecules and provide rapid deposition rates at much reduced temperatures.
Silane is used for the deposition of silicate glasses when substrate topography is minimal, as the deposited layers are characterized by poor conformality and poor step coverage. Silicate glasses deposited from the reaction of TEOS with O2 or O3 are being used with increasing frequency as interlevel dielectrics because the deposited layers demonstrate remarkable conformality that permits the filling of gaps as narrow as 0.25 xcexcm. Unfortunately, the deposition rate of silicate glass formed by the reaction of TEOS and O3 is highly surface dependent. A particularly acute problem arises when the deposition is performed on a surface having topographical features with non-uniform surface characteristics. For example, the deposition rate is very slow on PECVD-TEOS glass layers, considerably faster on silicon and on aluminum alloys, and faster still on titanium nitride, which is frequently used as an anti-reflective coating for laser reflow of aluminum alloy layers. A correlation seems to exist between the quality and relative deposition rate of ozone-TEOS glass layers. For example, ozone TEOS glass layers that are deposited on PECVD-TEOS glass layers have rough, porous surfaces and possess high etch rates.
In U.S. Pat. No. 5,271,972 to K. Kwok et al., it is suggested that the surface sensitivity of ozone-TEOS glass layers deposited on PECVD-TEOS glass layers may be related to the presence of a hydrophilic surface on the PECVD-TEOS glass layers. A hydrophilic surface on the PECVD-TEOS glass layer may be attributable to embedded elemental carbon particles which are formed as the TEOS precursor gas is attached by oxygen radicals generated by the plasma. As elemental carbon particles are characteristically hydrophilic, they repel TEOS molecules, which are characteristically hydrophobic, and interfere with their absorption on the surface of the deposited layer. Thus, the poor absorption rate of TEOS molecules on the surface of PECVD-TEOS glass results in slowly deposited, poor-quality films. Experimental evidence indicates that deposition rates are low for hydrophilic surfaces and high for hydrophobic surfaces. For example, titanium nitride, being highly hydrophobic, readily absorbs TEOS molecules on its surface, which accelerates the deposition reaction.
Given the surface-dependent variation in deposition rates, it is not uncommon for ozone-TEOS glass layers to build up rapidly around aluminum conductor lines and much more slowly on PECVD glass layers on which the conductor lines are fabricated, thereby forming cavities of tear-drop cross section between adjacent conductor lines. FIG. 1 is a cross-sectional view which depicts the undesirable result obtained by conventionally depositing an ozone-TEOS layer 11 over aluminum conductor lines 12 which overlie an underlying PECVD-TEOS glass layer 113. Prior to patterning, the aluminum conductor lines 12 were covered with a titanium nitride layer which served as an anti-reflective coating during a laser reflow operation. A titanium nitride layer remnant 14 is present on the upper surface of each aluminum conductor line 12. A cavity 15 having a teardrop-shaped cross section has formed between each pair of aluminum conductor lines 12. Cavities in an interlevel dielectric layer are problematic primarily because they can trap moisture when the deposited glass layer is subjected to a planarizing chemical mechanical polishing step during a subsequent fabrication step. The moisture, if not completely removed prior to the deposition of subsequent layers, can corrode metal conductor lines during normal use and operation of the part, or it may cause an encapsulated integrated circuit device to rupture if the steam generated as the device heats up is unable to escape the package.
In U.S. Pat. No. 5,271,972, a technique is disclosed for improving the film quality of ozone-TEOS glass layers deposited on PECVD-TEOS glass layers. The method involves depositing the underlying PECVD-TEOS layer using high pressure and a high ozone-to-TEOS flow rate. For the last several seconds of the plasma-enhanced deposition process, a stepwise reduction in reactor power is carried out. It is claimed that this technique produces an interstitial silicon dioxide layer at the surface of the PECVD-TEOS layer which greatly reduces the surface sensitivity of subsequently deposited ozone-TEOS oxide layers.
This invention provides an alternative method for depositing highly conformal silicate glass layers via chemical vapor deposition through the reaction of TEOS and O3 and for minimizing surface effects of different materials on the deposition process.
The entire method, which can be performed in a single cluster tool or even in a single chamber, begins by placing an in-process integrated circuit having multiple surface constituents in a plasma-enhanced chemical vapor deposition chamber. A xe2x80x9ccleanxe2x80x9d silicate glass base layer that is substantially free of carbon particle impurities on an upper surface thereof is then formed on the wafer surface in one of two ways.
The first way of forming the clean base layer employs plasma-enhanced chemical vapor deposition using TEOS and diatomic oxygen gases as precursors to first deposit a xe2x80x9cdirtyxe2x80x9d silicate glass base layer having carbon particle impurities imbedded on the upper surface. Glass layers deposited via PECVD by the reaction of TEOS and O2 tend to have elemental carbon particles embedded therein. As these particles may impart hydrophilic surface characteristics to the deposited glass layer which may interfere with the subsequent deposition of dense, high-quality ozone-TEOS glass layers, the base glass layer is subjected to a plasma treatment which involves flowing a mixture of an oxygen-containing diamagnetic oxidant, such as ozone or hydrogen peroxide or a combination of both, and diatomic oxygen gas into the chamber and striking an RF plasma at a power of 50-350 watts for a period of from 30-300 seconds. It is hypothesized that the plasma treatment burns off the carbon particle impurities that are present on the surface of the dirty silicate glass base layer, thereby reducing the hydrophilic surface characteristics. The plasma treatment also creates a high degree of surface uniformity on the PECVD-deposited O2-TEOS glass layer.
The second way of forming the base layer involves flowing hydrogen peroxide vapor and at least one gaseous compound selected from the group consisting of silane and disilane into the deposition chamber. As an optional step, the clean base layer formed via this second method may be subjected to a plasma treatment identical to that performed on the dirty PECVD-deposited O2-TEOS glass layer. This optional plasma treatment step is performed merely to improve surface uniformity, not reduce hydrophilic surface characteristics.
Following the formation of the clean base layer, a final glass layer is deposited over the PECVD-deposited glass layer using chemical vapor deposition and TEOS and ozone as precursor compounds.